THE ATMOSPHERE. 81 



fined, that the quantity thus conveyed one year with the other is 

 nearly the same : for that is the quantity which we see running 

 down to the ocean through these rivers ; and the quantity dis- 

 charged annually by each river is, as far as we can judge, nearly 

 constant. 



We now begin to conceive what a powerful machine the at- 

 mosphere must be ; and, though it is apparently so capricious and 

 wayward in its movements, here is evidence of order and arrange- 

 ment which we must admit, and proof which we can not deny, 

 that it performs this mighty office with regularity and certainty, 

 and is therefore as obedient to law as is the steam-engine to the 

 will of its builder. 



119. It, too, is an engine. The South Seas themselves, in all 

 their vast inter-tropical extent, are the boiler for it, and the north- 

 ern hemisphere is its condenser. 



120. Where does the vapor that makes the rains lohichfeed the 

 rivers of the northern heinisphere come from ? 



The proportion between the land and water in the northern 

 hemisphere is very different from the proportion between them in 

 the southern. In the northern hemisphere, the land and water are 

 nearly equally divided. In the southern, there is several times 

 more w^ater than land. All the great rivers in the world are in 

 the northern hemisphere, where there is less ocean to supply them. 

 Whence, then, are their sources replenished ? Those of the Ama- 

 zon are supplied with rains from the equatorial calms and trade- 

 winds of the Atlantic. That river runs east, its branches come 

 from the north and south ; it is always the rainy season on one 

 side or the other of it ; consequently, it is a river without periodic 

 stages of a very marked character. It is always near its high- 

 water mark. For one half of the year its northern tributaries are 

 flooded, and its southern for the other half. It discharges under 

 the line, and as its tributaries come from both hemispheres, it can 

 not be said to belong exclusively to either. It is suppUed with 

 water from the Atlantic Ocean. Taking the Amazon, therefore, 

 out of the count, the Rio de la Plata is the only great river of the 

 southern hemisphere. 



There is no large river in New Holland. The South Sea Isl- 

 ands give rise to none, nor is there one in South Africa that we 

 know of. 



F 



